Two general approaches typically are used to modulate the intensity of light: direct modulation and external modulation.
In a direct modulation approach, a laser (e.g., a laser diode) is directly modulated by an information signal to generate a modulated laser output. The laser output power often is modulated directly by modulating the input drive current to the laser. The laser begins lasing when the drive current exceeds a threshold current level. Typically, the modulation range of input drive current that is applied to a directly modulated laser extends above and near the threshold current level.
In an external modulation approach, a modulator modulates the intensity of light generated by a continuous wave laser in accordance with an information signal. The modulator and laser may be disposed on separate, discrete substrates or they may be fabricated together on a single substrate. External modulators fall into two main families: electro-optic type modulators, such as Mach-Zehnder type electro-optic modulators, which modulate light through destructive interference; and electro-absorption modulators, which modulate light by absorption (e.g., through the quantum-confined Stark effect).
Under direct modulation linear and nonlinear effects within the laser create chirp. Chirp is a variation in optical signal wavelength over the duration of a laser light pulse during modulation. For positive transient chirp, the leading edge of the laser light pulse comprises shorter wavelengths than the trailing edge. In positive dispersion fibers, shorter wavelengths travel faster than longer wavelengths. The pulse therefore broadens as it propagates. Regenerators often are required in order to compensate for this positive chirp, raising the cost of communications networks considerably. Chirp effects are manageable at direct laser modulation bit rates up to a few GHz. Direct modulation of lasers typically is not used at bit rates above a few GHz, especially when the laser is driven to create sharp laser pulses with abrupt rising and falling edges.
External modulation is favored for applications that are sensitive to chirp because external modulation introduces very little chirp into the output signal. For this reason, external modulation is used almost exclusively in long-distance digital optical communications, where excessive spectral broadening in a directly modulated laser due to chirp leads to a greater pulse distortion during propagation and a reduction in overall performance.
Distributed feedback (DFB) lasers are typically used for long-distance optical communication applications. A DFB laser produces an output that is characterized by a narrow spectral linewidth, which allows a DFB laser to transmit signals over long distances. This feature also allows a DFB laser to be used in narrow-linewidth applications, such as wavelength-division multiplexing (WDM) where it is desirable to carry as many multiplexed signals as possible without interference in the same optical fiber. DFB lasers, however, are extremely sensitive to back-reflections, which broaden the spectral linewidth and increase noise. For this reason, DFB lasers typically are assembled in one package with an optical isolator that blocks back-reflections.
The narrow linewidth features of DFB lasers and the low chirp characteristics of external modulators are leveraged in long-haul optical data transmission systems. The output wavelength temperature coefficient of a DFB laser and the absorption edge wavelength coefficient of an electroabsorption modulator, however, typically are significantly different, which degrades operation over wide temperature ranges. For this and other reasons, systems that include DFB lasers and electroabsorption modulators also typically include direct active temperature-regulating devices, such as thermoelectric coolers. In one such approach, a DFB laser and an electroabsorption modulator are mounted on an optical platform that is mounted on a submount, which is attached to a thermoelectric cooler. A thermistor mounted on the submount provides thermal feedback that allows the thermoelectric cooler to maintain the temperature of the DFB lasers and the electroabsorption modulators within a prescribed narrow temperature range.
For the reasons explained above, DFB laser designs tend to be bulky, expensive, and high in power consumption.